Imaging systems for electromagnetic radiation, particularly that portion of the radiation spectrum corresponding to visual and infrared radiation, are widely used, for example, for imaging objects at long distances by telescope. It is generally recognized that imaging systems require a minimum of three optical elements in order to provide the minimum number of parameters which are necessary to correct for spherical aberration, coma, astigmatism and field curvature. A telescope or imaging system which is comprised of three optical elements is generally known as a triplet. One common type of triplet is constructed using reflective optical elements, and is typically comprised of a negative powered mirror interposed between two positive powered mirrors. In a conventional reflective triplet light entering the system from a distant object first impinges on a primary mirror, is reflected onto a secondary mirror, is then further reflected to a tertiary mirror and finally is focused to an image plane where an image of the distant object is formed.
U.S. Pat. No. 4,420,707 to Wetherell et al. describes an all-reflecting, eccentric field, non-relayed optical system that is comprised of a reflective triplet consisting of a negative convex secondary mirror positioned between a positive concave primary mirror and a positive concave tertiary mirror, and has an aperture stop on the optical axis, the aperture stop being physically located on the secondary mirror. The entrance pupil of this optical system is virtual, that is, it is located a large distance behind the optical system. A disadvantage of having a virtual entrance pupil is that a significant amount of beam wander will occur in front of the optical system. Such beam wander detrimentally affects the image quality in situations requiring the viewing of objects through a small port or window. However, as a natural consequence of the aperture stop location on the secondary mirror, all of the imaging f-cones are substantially normally incident on the image plane (the telecentric condition).
U.S. Pat. No. 7,733,955 to Cook describes a reflective triplet including a positive concave primary mirror, a negative convex secondary mirror and a positive concave tertiary mirror, the mirrors being arranged such that the system formed thereby is used off-axis in both aperture and field angle. Positioned in front of the primary mirror is an entrance pupil for admitting into the optical system the radiation to be imaged. With this pupil location, there is essentially no beam wander at this point, and this is an advantage in viewing objects through a small port or window. However, as a natural consequence of the front aperture stop location, the imaging f-cones will exhibit a variation of incident angles on the imaging plane (departure from the telecentric condition), and this can be disadvantageous in certain circumstances or applications.